Biomedical studies with vanadate (VO4/3) and its interaction with Na+ - K+-ATPase, are very important in the research dealing with the exchange of sodium and potassium cations through the cytoplasmic membrane. The activity of this enzyme is related to a portion of the Na+ reabsorbed in the kidney. Vanadate has been shown to be a potent inhibitor of renal tubular Na+ -K+ -ATPase activity. The infusion of ouabain and vanadate (both Na+ -K -ATPase inhibitors) instead of potentiating the vascular effects of each other provokes paradoxical vasodilation. The observed effect could partly be interpreted by an interaction between vanadate (VO4/3-) and ouabain through oxidation reduction process and further complex formation between vanadyl (VO2+) and ouabain. The possibility of observing the effect of both inhibitors directly on the enzyme and the establishment of how an intermediate is involved in an enzymatic reaction is another route of this study that will be intended. These studies are performed in solution and in proteoliposomes. The vanadium species or oxidation state IV, (VO2+) or V, (VO4/3-) that may be responsible for the vasodilator effect will be studies by electrochemical methods. Direct observation of the effect of the inhibitors on the enzyme will be monitored either by 51V and 31P-NMR or by the enzyme's phosphorylating activity determined by well known method. The results should establish any difference in the enzyme's intermediate(s) that is(are) involved in the enzymatic reaction in the presence of the inhibitors.